Field of the Invention
The present invention relates to a novel intake channel arrangement for a volute casing of a centrifugal pump, a flange member, a volute casing for a centrifugal pump and a centrifugal pump. The present invention relates especially to a novel volute casing producing a substantially constant suction specific speed for different pumps of a centrifugal pump series.
Background Art
The main components of a centrifugal pump having an influence on the pumping characteristics thereof are an impeller, a volute casing, and especially, an intake channel thereof leading the medium to be pumped to the impeller. There are basically three types of impellers. A so-called open impeller, is generally formed of a hub and working vanes attached to the hub. The hub is provided with a central hole for fastening the impeller to the shaft of the pump. If the hub is extending radially outwardly by a so-called rear plate or shroud to which the working vanes are arranged at their rear edges, the impeller is called a semi-open impeller, i.e. the front edges of the working vanes being free or open. If the front edges of the working vanes are fastened to a plate, so-called front plate or shroud, too, the impeller is called a closed impeller.
The volute casing comprises normally an intake channel, a front wall following, in the flow direction of the medium to be pumped, the intake channel and continuing radially outwardly, substantially following the shapes of the front edges of the working vanes or front shroud of the impeller, and a volute. Normally, a cross-section of the volute in an axial plane increases in a circumferential direction of rotation of the impeller up to a discharge outlet opening or a pressure outlet which is normally more or less tangential. The volute casing is fastened to a rear wall or a casing cover of the pump, and forms together with the rear wall or the casing cover of the pump a chamber or a cavity designed to house at least an impeller being usually of the radial or mixed flow type and mounted on a shaft for rotation when driven by a motor. The shaft is supported within a pump casing by bearings and a sealing such as a mechanical seal or packing box is provided for sealing the shaft in relation to the pump casing.
The impeller rotates around an axis of rotation in the pumping cavity formed between the front wall, the volute and a back or rear wall of the pump so as to pump the medium and to discharge the medium from the pump via the pressure outlet or the discharge duct. The discharge duct can be arranged tangentially to the volute casing or arranged radially by providing a so-called swan neck. The point where the discharge flow separates from the flow continuing its circulation in the volute casing is called a cutwater. Centrifugal pumps are usually single stage pumps but two stage and multistage pumps are also in use in some applications.
There are two common volute casing types, i.e. a single suction type and a double suction type. In the case of the single suction type, the liquid is drawn from one axial side of the pump, and is pumped radially/tangentially out of the pump. In the double suction type, the pump draws the liquid from both opposite axial sides of the pump, and pumps the liquid radially/tangentially out of the pump.
Since a centrifugal pump can be designed to work optimally only at a certain substantially narrow performance (head, flow rate) range, each pump manufacturer designs a series of pumps (see FIG. 8) such that a user is able to find a suitable pump for all his/her pumping needs. Such a series of pumps has the same basic design, only the dimensions of the volute casings and the impellers are changed, i.e. the basic pump is scaled to a number of different sizes.
When a centrifugal pump is connected to the inlet pipeline, there is, almost always, a difference in diameters between the inlet pipeline and the inlet opening at the borderline between the intake channel and the front wall of the pump introducing the medium to be pumped to the effective area of the impeller. The difference in diameters is due to two facts: 1) metal pipes used for transferring pumpable media in industrial processes are manufactured in accordance with international pipeline standards, and 2) the performance requirements of the centrifugal pump, i.e. the desired head and flow rate dictate the diameter of the inlet opening of the centrifugal pump. As the dimensioning of the centrifugal pump, including the calculated diameter of the inlet opening, is designed to be optimal for the desired head and flow rate it is very seldom that the diameter of the inlet opening happens to match that of the pipeline.
The two diameters are usually made to match by arranging an appropriate reduction or increase in the diameter of the pump intake channel such that the diameter at the first end of the intake channel, i.e. that of the inlet flange, matches to the diameter of the inlet pipeline and the diameter at the second end of the intake channel to the calculated diameter of the inlet opening. Therefore, it has been common practice to form a substantially conically shaped intake channel in the volute casing in front of the impeller. When the intake channel is converging in the direction of flow, the flow is accelerated before its introduction to the effective area of the impeller. And when the intake channel is diverging in the direction of flow, the flow is decelerated before its introduction to the effective area of the impeller. In both cases, flow losses are created, though in the latter case the losses are significantly higher than in the former case. The magnitude of the losses depends on the dimensioning of the conical intake channel. A pump series thus consist of different sizes of pumps wherein the flow is accelerated in some pumps and decelerated in some other pumps before its introduction to the effective area of the impeller. It is important for the user of the pump to know the magnitude of the flow losses of the pump to be able to choose a tight pump for his/her applications. Since the flow losses of the pump itself are very well known, it is the changing or varying design of the suction or intake channel that forms a problematic and hard to predict source of flow losses.
Suction specific speed (NSS) is a parameter used in characterizing the operation of a centrifugal pump. It is mainly used to see if there will be problems with cavitation on the suction side during the pump's operation. In practice, the shape and dimensioning of the intake channel have a significant impact in the actual value of the NSS. The suction specific speed is discussed in more detail in, for instance, http://www.pumpingmachinery.com/pump_magazine/pump_articles/article_03/article_03.htm. The value for the NSS can be calculated by
      NSS    =                            N          ⁡                      [            rpm            ]                          ·                              Q            ⁡                          [                                                m                  3                                ⁢                                  /                                ⁢                s                            ]                                                            (                      NPSHR            ⁡                          [              m              ]                                )                0.75              ,where N is a rotational speed (revolutions per minute), Q is a pump capacity (cubic meter per second) and NPSHR is a net positive suction head required by the pump (meter) that is normally calculated at the best efficiency point (BEP). As can be seen, the NSS considered herein is calculated in SI units.
Thus, each pump has its characteristic NSS. And, naturally, the NSS's of all pumps or pump sizes of a pump series should be as close to each other as possible. In case there are significant deviations in the NSS's of different pumps or pump sizes, it will be difficult to determine which pump is optimal for a certain application. For instance, if the NSS of a certain pump size is lower than that of the other pump sizes, it means that the suction head is higher, whereby the pump in question cannot be used in an application requiring a low suction head, but a larger, and more expensive pump has to be chosen.
When using a conically shaped intake channel to match the centrifugal pump to the inlet pipeline, the intake channel will affect the suction specific speed of more or less all pump sizes in a pump series, as the conical intake channels of different pump sizes have (most probably) different dimensions. The basic reason for such deviations in the NSS is the fact that the losses generated by the conically shaped intake channels vary depending on the design of the cone. In accordance with performed calculations the suction specific speed of centrifugal pumps of a prior art pump series varies ±5-7% around the average NSS value, i.e. the total variation being 10 to 14%. It means, in practice, severe difficulties in determining which pump is ideal for the customer's application.